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Discussion Paper
Drink. Water Eng. Sci. Discuss., 8, 177–196, 2015
www.drink-water-eng-sci-discuss.net/8/177/2015/
doi:10.5194/dwesd-8-177-2015
© Author(s) 2015. CC Attribution 3.0 License.
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M. F. Mohd Amin1,2 , S. G. J. Heijman2 , and L. C. Rietveld2
Discussion Paper
Clay-biodegradable polymer combination
for pollutant removal from water
Correspondence to: M. F. Mohd Amin ([email protected])
Published by Copernicus Publications on behalf of the Delft University of Technology.
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Received: 30 July 2015 – Accepted: 3 September 2015 – Published: 23 September 2015
Clay-biodegradable
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from water
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Faculty of Earth Science, Universiti Malaysia Kelantan, Jeli Campus, Locked Bag 100, 17600
Jeli, Kelantan, Malaysia
2
Department of Sanitary Engineering, Faculty of Civil Engineering and Geosciences, Delft
University of Technology, P.O. Box 5048, 2600 GA Delft, the Netherlands
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The wastewater chain is an important source of priority substances into the surface
water system and is, therefore, one of the bottlenecks in achieving the European
Water Framework Directive objectives (Directive 2001/83/EC, amended by Directive
2004/27/EC (for human pharmaceuticals), and Directive 2001/82/EC, amended by
2004/28/EC (for veterinary pharmaceuticals). At various locations in the Netherlands
the standards for priority substances are exceeded. The current concern in detecting
these micropollutants in receiving waters may call for new approaches in wastewater
treatment (Mohd Amin, 2015). The introduction of an economical and effective method
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Introduction
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In this study, a new treatment alternative is investigated to remove micropollutants from
wastewater effectively and in a more cost-effective way. A potential solution is the use of
clay in combination with biodegradable polymeric flocculants. Flocculation is viewed as
the best method to get the optimum outcome from the combination of clay with starch.
Clay is naturally abundantly available and relatively inexpensive compared to the conventional adsorbents used. Experimental studies were carried out with four different
clays to select the best clay for further optimisation. The atrazine removal achieved is
−1
in the range of 10–99 % based on the clay concentration of 10–50 g L . Optimisation
of the best clay performer leads towards atrazine reduction of > 99 % with a dosage
−1
of 100 mg L . The best and underperforming clays were then tested in other experiments with the addition of cationic starch flocculants. In this experiment, the addition
of a polymer increased the atrazine removal for the underperforming clay to 46 % with
−1
only 10 mg L clay dosages. The clay flocculation test was also performed to test the
flocculation efficiency of clays by the polymer. Approximately 80–84 % of the clay is
flocculated, which shows exceptional flocculation efficiency in removing both clays and
atrazine from the water matrices.
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in removing these compounds from wastewater is therefore crucial (Virkutyte et al.,
2010).
The availability of advanced treatment methods has improved the removal of these
micropollutants from wastewater and existing conventional wastewater treatments can
be upgraded (Ternes, 1998; Stumpf et al., 1999; Heberer, 2002). However, such treatment is often costly. A potential low-cost solution is the use of natural substances for
micropollutant removal (Radian and Mishael, 2012), such as clay in combination with
biodegradable polymeric flocculants. The application of biodegradable polymers such
as cationic starch (CS) to flocculate particles during treatment has the additional advantage of being shear resistant (Singh et al., 2000).
In the last few decades, interest in the adsorption of polyelectrolytes on clay surfaces for enhanced removal of pollutants has grown (Ternes, 1998; Singh et al., 2000).
For example, Churchman (2002) demonstrated the removal of toluene by polystyrene–
montmorillonite (MMT) composites. Radian and Mishael (2012) showed that, at high
loadings of PDADMAC on MMT, the composite is positively charged, promoting the
binding of anionic herbicides. In a recent study, the advantages of composites of poly4-vinylpyridine-co-styrene (PVPcoS) and MMT for the removal of atrazine from water, even in the presence of dissolved organic matter (DOM), were reported (Zadaka
et al., 2009).
The combination of clays and polymers can be viewed as a symbiosis interaction, because organic micro-pollutant absorbing clays need to be settled to avoid being flushed
out. For that, the dosage of a cationic polymer can be beneficial due to its ability in particle removal (Van Nieuwenhuijzen, 2002). Cationic polymers have also been proven to
be efficient in e.g. atrazine reduction from water matrixes, but it requires attachment to
a negatively charged surface (Mohd Amin et al., 2014a). The combination of cationic
polymers with clays is thus hypothesised to enhance the micro-pollutant reduction by
creating a diffuse zone (Eslinger and Pevear, 1988; Mohd Amin et al., 2014c).
Clay such as smectite has a high cationic exchange capacity (80–150 meq (100 g)−1 )
and can also act as a coagulant aid in wastewater treatment for destabilisation of the
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other particles in water before floc formation with polymeric flocculants occurs (Sawhney and Singh, 1997; Mohd Amin et al., 2014b). This paper reports on the direct interaction between clays and a CS with atrazine as a model compound in demineralised
water. Four different types of clay were compared and the best performer was selected
for optimisation. The flocculation aspect of the clays with CS was also examined in
order to find the combined dosage for optimal performance.
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Clay selection and optimisation
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Experimental studies were carried out with four different clays, which were selected
based on their different properties. The attapulgite (ATT), smectite (SME), and sepiolite (SEP) were supplied by Tolsa Group (Spain) through Keyser & Mackay (the
Netherlands) while Na-Bentonite (BEN) was purchased from Sigma-Aldrich. All the
clays consist of a ∼ 1–4 nm thick surface layer and are around ∼ < 1–5 µm-sized. The
Nalco cationic starch EX10704 was used and obtained from Nalco Netherlands BV.
®
Atrazine (PESTANAL , analytical standard) and analytical grade methyl tertiary butyl
ether for gas chromatography measurements were purchased from Sigma-Aldrich.
Properties of the clays and the polymer are listed in Tables 1 and 2. Demineralised
water was obtained from tap water that was treated with reverse osmosis and ion exchange. Whatman Spartan 30/0.45 RC syringe filters (0.45 µm, Whatman UK) were
used to filtrate the samples as pre-treatment.
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Material and methods
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Clay selection experiments were prepared by adding a range of clay dosages (0–
−1
−1
50 g L ) to a 200 mL solution with a concentration of 6 ± 2 µg L atrazine in a 500 mL
Duran glass bottle. The solutions with the clay were stirred at 40 rpm for 24 h before
settling took place for 2 h. After the experiment, the solution was filtered with the sy-
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ringe filter (0.45 µm). The collected samples were, after filtration, pre-treated before
being analysed for atrazine residues.
The best clay (i.e. SME) from the first experiment was optimised for low dosages to
−1
adsorb an initial atrazine concentration of 2±0.2 µg L (normally found in wastewater).
The solutions with the clay were stirred at 40 rpm for 8 h before settling took place for
1 h. The rest of the procedure was similar as aforementioned.
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SME-CS: atrazine removal, flocculant dosage and turbidity relation
The objective of this experiment was to further optimise the flocculation of the clays
with CS. The flocculation experiments of the SME using CS (no atrazine added) were
carried out in jar test equipment filled with demineralised water (1.8 L). All the jars
contain four baffles to increase the energy gradient during stirring. Figure 1 illustrates
the specific relationship between the energy gradient (G value) and the rpm of the
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The clay (SME, ATT) combination with CS was studied to determine the effect on
atrazine reduction. The experiments were carried out in a pyrolysed 2000 mL Duran
glass bottle. Both clays in concentrations in the range 0–120 mg L−1 (based on previous
optimisation experiments) were first dosed to adsorb the atrazine (initial concentration
of 2 ± 0.2 µg L−1 ) for 8 h. For ATT the adsorption time applied was 24 h, to ensure that
equilibrium between atrazine and the clays was reached before flocculation. Then the
−1
CS with a concentration of 20 mg L was added and mixed slowly at 40 rpm for 2 h before settling for 1 h. Extended agitation was applied to produce smaller flocs resulting
in a higher surface availability especially for atrazine removal by ATT. Samples were
taken, filtered with the syringe filter (0.45 µm) and analysed.
ATT in combination with CS was further tested with different starch concentrations
−1
(10, 20, 40, 60 mg L ) to prove that the atrazine reduction is enhanced by the starch
concentration. The experimental procedure was similar as aforementioned.
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Clay flocculation with cationic starch
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A : the settled clay concentration
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B : total concentration of clay
C : percentage of atrazine removed by settled clay
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(A/B) · 82 % = C(%)
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stirrers for this jar tester. The clay dosage used in this experiment was in the range
of 0–100 mg L−1 and the CS concentration was fixed at 20 mg L−1 . Dosing of the CS
was conducted at a stirring velocity of 300 rpm. After a mixing time of 30 s, the stirring
velocity was reduced to 40 rpm for 10 min. During this period, floc formation took place.
All the jars were observed visually, and after a settling time of 20 min, samples were
taken for analysis. Turbidity was measured using a Hach DR5000 spectrophotometer
according to APHA (1999) standard methods.
The settled and suspended clay is measured based on the turbidity level of SME
−1
from 0–40 mg L dosage based on Fig. 2. The turbidity measurement will be taken
after each of the settling periods. The turbidity reading will then be compared to the
reading in Fig. 2 in order to determine the SME dosage. This measured SME dosage
represents the suspended SME in the water matrix. The rest of the SME in the water
matrices will have settled.
The atrazine concentration is determined by calculating the removal from the blank
−1
sample (quadruplicate) of SME (40 mg L ) and atrazine without any polymer addition.
On average, 82 % of atrazine is removed and the final turbidity after settling is approximately 5.5 NTU. The turbidity value was used for the determination of SME concentration based on Fig. 2. Based on this turbidity reading and Fig. 2, it can be deduced
−1
−1
that 35 mg L is suspended in the water matrixes. The rest of the 5 mg L of SME is
expected to settle. With this information, the amount of atrazine attached to the settle
clay can be calculated based on Eq. (1).
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The reduction of atrazine, with initial concentrations of 6 ± 0.2 µg L , for four different
−1
clays dosed in the range of 0–50 g L is shown in Fig. 3. Atrazine reduction from the
water was in the range of 10–99 % for three out of the four selected clays. There was
no reduction achieved with BEN. BEN has a hydrophilic surface, which attracts water
molecules for attachment due to the presence of sodium ions thus limiting the interaction with atrazine. For ATT, atrazine reduction was increased only at dosages larger
−1
−1
than 5 g L and reached a maximum of 40 % at 50 g L . It is reported that the surface charge of ATT is approximately negative to neutral (White and Hem, 1983), which
led to a low atrazine affinity at dosages lower than 5 g L−1 . This is supported by the
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−1
The atrazine concentrations (2 ± 0.2 µg L ) were analysed by gas chromatography
(Agilent’s 7890A) based on the U.S. Environmental Protection Agency 551.1 (1995)
method. Atrazine in the sample was extracted using liquid–liquid micro-extraction with
MTBE as a solvent. The injection sample was 1 mL of the extracted sample. A volume of 2 µ L−1 was injected in splitless mode and the injector temperature was 200 ◦ C.
−1
The carrier gas was helium (linear velocity was 33 cm s ). The injector temperature
◦
◦
was 260 C. The oven temperature was held at 35 C for 9 min, and then raised at
◦
−1
◦
◦
15 C min intervals to 225 C. The temperature of 225 C was held for 10 min before
◦
−1
◦
being raised at 20 C min intervals to 260 C. The recovery of atrazine was in the
range of 90–110 %.
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The rest of the sample and replicate were measured and calculated based on this
equation. The atrazine removal percentage in the solution is the remainder that is removable by clay.
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Atrazine reduction from water with SME was 45–99 % before the addition of CS. After
the CS addition, the reduction increased at low clay dosages (Fig. 4). The largest effect
of the CS addition in enhancing the atrazine reduction was observed at SME dosages
−1
of 10–40 mg L . The high reduction is expected to be due to the attachment of CS
that covers the entire clay surface during floc formation, thus creating a diffuse zone
that is able to trap atrazine (Mohd Amin et al., 2014a). At an SME dosage higher than
40 mg L−1 , the clay’s own ability in adsorbing the atrazine predominates, thus resulting in a higher removal than without the addition of CS. The ATT alone did not have
much ability in reducing the atrazine concentration at low dosages as shown in Fig. 3.
However, after the addition of CS, the atrazine reduction was considerably increased,
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low cation exchange capacity (CEC) value of ATT being around 20–30 meq (100 g) .
Similar observations were also reported by Haden (1961). SEP showed a constant increase in removal, with 10 % atrazine reduction at a clay dosage of 0.05 g L−1 to > 99 %
at a dosage of 25 g L−1 . The ability of SEP in adsorbing organic compounds has previously been reported by Rytwo (2012). SME showed the highest affinity to atrazine
−1
removal with reductions > 99 % at dosages lower than 1 g L . SME has the highest
2 −1
−1
specific surface area (200–800 m g ) and the highest CEC (80–150 meq (100 g) )
value compared to the other three clays. It was also reported that SME can adsorb
other compounds, such as carbamazepine (Zhang et al., 2010) and naphthalene and
phenanthrene (Lee et al., 2004). SME was thus viewed as the most suitable clay for
further investigation.
The SME as the best clay and ATT as the low performer reference clays were further
−1
tested for optimal dosages in atrazine (initial 2 ± 0.2 µg L ) adsorption. In this experiment, more than 86 % atrazine was removed using 60 mg L−1 SME and this reached
a maximum of > 99 % at dosages up to 100 mg L−1 . Therefore, we limited the dosage
−1
in the following experiment to clay dosages below 100 mg L .
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In this experiment, the relation between CS dosage and clay is further studied to find
a relation between the atrazine reduction, CS dosage and SME turbidity. Atrazine in
the water matrix can be divided into three different phases after settling, suspended
(adsorbed to SME but does not settle) and remaining in the solution (not adsorbing).
In Fig. 6, it can clearly be seen that the SME had a high ability in atrazine adsorption,
which is reflected in the “suspended” phase. At 40 mg L−1 without any CS, around
62 % atrazine is adsorbed from the solution but suspended and not removed from
the wastewater matrix. By increasing the CS dosage, the amount of settled SME that
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specifically for the 10 mg L clay dosage with an atrazine reduction of 45 %. The reduction percentage decreased with the clay dosage increments, to 7.5 % at a dosage
of 120 mg L−1 . At a lower clay dosage (10 mg L−1 ), the polymer was expected to cover
the entire clay surface. With the clay dosage increment, more flocs were formed, and
fewer polymer surfaces were available for atrazine attachment. At higher clay dosages,
even more and larger flocs were formed, and less free surface was available which
resulted into a lower removal percentage.
From Fig. 4, it is observed that the clay dosages limit the atrazine reduction by ATT.
This limitation was further studied at CS concentrations from 10–60 mg L−1 (Fig. 5).
Best results were obtained at a clay dosage of 10 mg L−1 and a CS dosage > 20 mg L−1 .
The higher CS dosages did not have a large influence in increasing the atrazine reduction. A further increase in the ATT dosage resulted in a similar pattern. At a limited ATT
−1
concentration of 10 mg L , there was high competition for CS attachment. The extent
of CS attachment to the ATT layer is limited by the CS concentration, which translated
into different atrazine reduction levels with different CS concentrations. It can also be
that, when the ATT dosage increases, a higher surface availability results in less CS
multilayer formation and thus in less atrazine diffusion, which leads to less atrazine
reduction.
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The present study was designed to determine the ability of different clays in combination with a CS to reduce atrazine in water. SME, as the best performing clay, was
further optimised to lower the dosage based on atrazine concentrations regularly found
−1
in the environment. The effective SME dosage was around 20 mg L to reduce about
60 % atrazine from demineralised water, and a maximum of > 90 % atrazine reduction was reached with a clay dosage > 80 g L−1 . A combined dosage of SME and CS
was performed to study the effect of polymers on the clay’s ability in the reduction of
atrazine.
Efficient flocculation by addition of CS increases the settled SME and simultaneously
increases atrazine reduction from the water. However, here 20 mg L−1 CS dosage was
viewed as sufficient for atrazine reduction through the settling of clay (72 %) with around
82 % of total turbidity removal. Application of higher CS dosages, although improving
the turbidity removal and settling the clay, did not significantly improve the total atrazine
reduction. It should be noted that the application of inorganic matter such as clay in
some countries like the Netherlands is undesirable due to increased waste production, which will increase the disposal cost. However, in certain circumstances, the high
sludge productions are also preferable for the production of biogas and can even be
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contains atrazine increased from 10 % without any CS dosage to a maximum of 77 %
with 30 mg L−1 dosage. The amount of suspended SME (approximately 2.5 mg L−1 )
−1
was at the lowest at 40 mg L CS, which accounted for 5 % atrazine removal.
Figure 6 also shows that a further increase in CS dosage did not improve the atrazine
reduction although a higher settled clay percentage was observed. The maximum
atrazine reduction was 82 % at 30 mg L−1 CS and was slightly reduced (80 %) at higher
dosages. The turbidity removal reached a maximum at 40 mg L−1 CS with around 92 %
removal. However, the amount of SME settled was slightly less compared to the settled
−1
SME at 30 mg L CS dosage.
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Churchman, G. J.: Formation of complexes between bentonite and different cationic polyelectrolytes and their use as sorbents for non-ionic and anionic pollutants, Appl. Clay Sci., 21,
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Acknowledgements. Author’s thanks Nalco Netherlands B.V especially to Sonia Lopez; Waterboard Rijnland (the Netherlands) and Agentschap (the Netherlands) for research grant
(Optimix–Project); Tolsa Group (Spain) and Keyser & Mackay (the Netherlands) for the Tolsa
clay product supply. Amer El-Khalily for the analytical advice. The scholarship given by the
Malaysian Ministry of Higher Learning (KPT, Malaysia), and Universiti Malaysia Kelantan (UMK,
Malaysia), which provided financial support, for the author PhD study is gratefully acknowledged.
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used as fertilizer in some countries. An extended study of the applications of the claypolymer combination in terms of its performance, reuseability, impact and economic
value, is required for more accurate information.
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Mohd Amin, M. F., Heijman, S. G. J., and Rietveld, L. C.: Nanoclay for micropollutant removal in wastewater – effective alternative?, Adv. Mater. Res., 1024, 11–14,
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Zadaka, D., Nir, S., Radian, A., and Mishael, Y. G.: Atrazine removal from water by polycationclay composites: effect of dissolved organic matter and comparison to activated carbon,
Water Res., 43, 677–683, doi:10.1016/j.watres.2008.10.050, 2009.
Zhang, W., Ding, Y., Boyd, S. A., Teppen, B. J., and Li, H.: Sorption and desorption of carbamazepine from water by smectite clays, Chemosphere, 81, 954–960,
doi:10.1016/j.chemosphere.2010.07.053, 2010.
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200–800
80–150
Hydrated
magnesium
aluminium
silicate
150
Pangel s9
Magnesium
silicate
∼ 320
4–40
∼ 8–9
30–90
Na-Bentonite
Aluminium
silicate
220–270
∼ 40
6–9
600–1100
Smectite
(SME)
Minclear N100
Attapulgite
(ATT)
Minclear N300
Sepiolite
(SEP)
Bentonite
(BEN)
20–30
∼ 8.6
∼ 9.5
Bulk
density
−1
(g L )
500–600
310–430
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Hydrous
magnesium
silicate
Composition
Clay-biodegradable
polymer combination
for pollutant removal
from water
Discussion Paper
pH
Cation
exchange
capacity
(CEC)
(meq (100 g)−1 )
Commercial
name
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Surface
area
(m2 g−1 )
Clay
mineralogy
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Table 1. Properties of clays used in this study.
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Solubility
(in water)
Ionic
character
Molecular
weight
Nalco
StarchEX10704
(CS)
Modified potato
starch
Flaked
solid
Soluble
Cationic
10 –10
6
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Form
Clay-biodegradable
polymer combination
for pollutant removal
from water
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Description
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Product
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Table 2. Properties of the polymer used in this study.
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300
0
0
50
100
150
200
Number of revolutions (rpm)
Figure 1. Mixing energy for the jar tester.
250
300
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100
Clay-biodegradable
polymer combination
for pollutant removal
from water
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200
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G-value (s-1)
|
400
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6
Discussion Paper
R² = 0.9813
5
4
1
0
0
5
10
15
20
25
SME Dosage mg/L
30
35
40
Figure 2. Initial turbidity mapping of SME in demineralised water based on the dosage 0–
40 mg L−1 .
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2
Clay-biodegradable
polymer combination
for pollutant removal
from water
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3
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Initial turbidity (NTU)
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7
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100
80
60
0
0.01
0.1
Smectite
1
Clay dosage (g/L)
Sepiolite
Attapulgite
10
Na-Bentonite
100
Figure 3. Atrazine adsorption (initial concentration 6 ± 0.2 µg L−1 ) by different types of clay with
dosages in the range of 50 mg L−1 –50 g L−1 .
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20
Clay-biodegradable
polymer combination
for pollutant removal
from water
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40
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Atrazine reduction %
Discussion Paper
120
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80
60
40
0
20
40
Att +20mg/L CS
60
80
Clay dosage (mg/L)
Sme +20mg/L CS
Sme
100
Att
120
Figure 4. Atrazine reductions by combination of clay and CS (20 mg L−1 ) with a dosage range
of 10–100 mg L−1 ; and with SME and ATT as a reference.
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0
Clay-biodegradable
polymer combination
for pollutant removal
from water
Discussion Paper
20
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Atrazine reduction %
|
100
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40
30
20
0
10
Att
20
10mg/L CS
30
40
50
Clay dosage (mg/L)
20mg/L CS
60
40mg/L CS
70
60mg/L CS
80
90
Figure 5. Atrazine reductions by combination of ATT and CS (10, 20, 40, 60 mg L−1 ) with
a dosage range of 10–80 mg L−1 .
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0
Clay-biodegradable
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Atrazine reduction %
|
50
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5
60%
4
50%
3
20%
10%
0%
1
0
2
Settled
5
10
20
CS dosage (mg/L)
Suspended
Solution
30
40
Turbidity
50
0
Figure 6. Atrazine reduction and turbidity removal for 40 mg L−1 SME with different CS dosages.
−1
Initial atrazine concentration 2 ± 0.2 µg L , initial turbidity 5.5 ± 0.2 NTU.
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30%
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40%
Clay turbidity (NTU)
70%
6
80%
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7
90%
|
Atrazine reduction %
Discussion Paper
100%
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